1. Technical Field
The present invention relates generally to systems for generating and using coherent light, and more particularly to determining and controlling the frequency of light used in such systems. It is anticipated that a primary application of the present invention will be in telecommunications, but the present invention is also well suited for use in laboratory measurement and other fields.
2. Background
The Fabry-Perot etalon has long been used for stabilizing laser frequencies, and the confocal etalon is now starting to see similar use. Due to its wide use, the Fabry-Perot etalon is used in the examples here, but it should be appreciated that the scope of what we present here is not limited to only that device. It should also be noted that the terms “frequency” and “wavelength” are used interchangeably in the following discussion.
In a typical fiber optics application, the spacing between the two reflectors of a Fabry-Perot etalon assembly is fixed and the resonant spectrum (transmissive or reflective) coincides with the ITU grids, which come in increments of 200 GHz, 100 GHz, 50 GHz, . . . , etc. This type of arrangement is termed a “wavelength locker” in the telecommunications industry.
FIG. 1 (background art) is a block diagram that conceptually shows the structures of two Fabry-Perot etalons that are commonly used in wavelength lockers for fixed wavelength applications. The first of these is etalon 10, an “air spaced etalon” as discussed below. It comprises two light transmissive plates 12 each having a partially reflective surface, i.e., reflectors 14. The reflectors 14 are separated apart a distance L1 by two spacers 16, thus defining a chamber 18 that contains a medium with a refractive index, n1.
The second Fabry-Perot device in FIG. 1 is etalon 20, a “solid etalon” as also discussed below. This comprises one light transmissive block 22 having two partially reflective surfaces, reflectors 24, separated apart a distance L2. The material of the block 22 is a medium having a refractive index, n2.
Numerous variations of Fabry-Perot etalons such as those in FIG. 1 are possible, for manufacturing convenience, etc. For example, the shape of the structure supporting the reflective surfaces can be either rectangular or round, and either two rectangular bars or a single cylinder can be used to separate the reflective surfaces.
FIG. 2 (background art) is a graph 30 showing a typical transmissive spectrum 32 of a wavelength locker. The relationship of frequency verses transmission intensity is depicted with a peak-valley curve 34, wherein adjacent peaks 36 define a free spectral range (FSR 38). For instance, 50 GHz.
FIG. 3 (background art) is a graph 40 showing, in simplified manner, the principle of a conventional wavelength locker using a Fabry-Perot etalon (e.g., etalons 10, 20).
When laser light is injected into the etalon its frequency falls somewhere on a peak-valley curve 42 that is characteristic for the particular etalon. The etalon is normally pre-calibrated so that this occurs in a shoulder region 44, typically centered about the 50% point with respect to amplitude on the ITU grids 46. The laser frequency of the wavelength locker is then normally adjusted to this 50% point, termed a lock point 48, and kept there by use of a servo control circuit. The laser frequency will thereafter remain stable as long as the peak-valley curve of the etalon does not drift.
In the etalons 10, 20 of FIG. 1, the spacings L1, L2 between the respective reflectors 14, 24 determine the FSR 38 of the resonant (transmissive or reflective) spectrum according to the equation:FSR=c/(2*n*L)  EQ. 1
where c is the speed of light in vacuum and n (n1 or n2 as the case may be in FIG. 1) is the refractive index of the medium between the respective set of two reflectors 14.
In the case of the air spaced etalon 10, when the medium between the reflectors 14, 24 is vacuum, n=1 and the only parameter that affects the FSR 38 is the spacing L1. When the medium between the paired reflectors 14 is air, n1˜1.000273 and the refractive index follows the Edlen equation (EDLEN, B., “The Refractive Index of Air,” Metrologia, 2, 71–80, 1966). An etalon of this type is generally called an “air spaced etalon,” regardless of whether the medium is vacuum, air, or some other gas mixture.
In the case of the solid etalon 20, when the block 22 is glass, n˜1.5 and the Fabry-Perot etalon effectively consists of a single piece of sold glass having both reflectively coated reflectors 24 parallel to each other. An etalon of this type is generally called a “solid etalon,” and the term “glass” may loosely mean any transparent solid medium.
The spacing L1, L2 between the reflectors 14, 24 is maintained constant so that the FSR 38 does not change during usage. This is achieved by using a material for the spacers 16 or block 22 (i.e., a medium) that has a low thermal expansion coefficient. Materials with such expansion coefficients are currently commercially available from Corning Glass™ in the U.S. and from Schott Glass™ in Germany (e.g., Zerodur™). These glass materials exhibit nearly zero thermal expansion in the environment typically required for telecommunications.
In addition to maintaining the spacing L constant, a process to keep the refractive index n constant has also been invented by Fibera, Inc. of Santa Clara, Calif. This process makes the wavelength locker “a thermal” and provides superior functionality throughout a very wide temperature range.
Such fixed spacing arrangements are fine, so long as the laser frequency does not have to be varied to achieve the underlying application. However, there are applications that require tuning the laser frequency through the ITU grids in a steady fashion, while also maintaining the frequency stability of the laser. A “tunable” wavelength locker would therefore be very useful for providing both frequency stabilization and tunability.
From in EQ. 1 it can be appreciated that tuning a wavelength locker can be achieved by varying either “L” or “n” in a controlled manner. First, consider tuning the FSR by varying “n.” This can be accomplished by changing conditions present in the wavelength locker package. From Eden's work, noted above, it is known that the refractive index of air is a function of pressure, humidity, and temperature. One of these parameters can therefore be precalculated and used to implement tuning. In actuality, however, this is not an easy process to accomplish. For example, the presence of a pressure adjusting device is usually not possible in the field.
Next, consider mechanically tilting the Fabry-Perot with respect to the incident laser beam, that is, effectively changing L. By doing this the optical path between the reflective surfaces is changed so that tuning is also achieved. However, this approach requires a motive means (e.g., a motor) to perform the tilting, and the addition of such a means to the wavelength locker is also undesirable in the field. For example, in the telecommunications field the constraints on space, with respect to both footprint and volume, can be quite severe. Recently there has been significant progress in MEMS technology, and tilting an etalon with a MEMS motor might be possible in the near future, but his does not address present needs.
Accordingly, there remains a need for a system to provide both frequency stabilization and tunability.